The sudden appearance of bats in the fossil record (approximately 50 million years ago) suggests that the evolution of the morphological specializations of bats for powered flight (i.e., elongation of the forelimb phalanges and reduction of the ulna) occurred very rapidly. This study tests the hypothesis that changes in limb morphology, as seen in the rapid evolution of bats, arise from changes in the temporal and spatial expression of a few key regulators of limb development. To test this hypothesis, the molecular and morphological patterning of the specialized limbs of a bat (Carollia perspicillata) will be compared with those of a mouse (Mus musculus) that closely resemble the limbs of the pre-flight bat ancestor. By illustrating the morphological and molecular basis of the evolution of the bat wing, this study will provide a better understanding of the relationship between gene expression patterns and the resultant morphologies and expand our knowledge of the natural variation of the roles of these genes. In addition, understanding the normal roles of genes that also play a role in abnormal developmental events such as cancer (i.e., Ihh and BMP) will provide a knowledge base for additional healthcare-related biomedical research.